Rotate part of object
To rotate an object, press the key 'r', and then press either 'x', 'y', or 'z' to get the rotation in a specific axis (This is not required. You can press 'r' and rotate freely.). The rotation will be based on the origin of the object, or the tiny yellow dot that shows up when you select it. In your case, if the origin is not at the base of the wing, follow these steps:


'Tab' to enter Edit Mode
Select the bottom face of the wing
Press 'Shift + S' and choose 'Cursor to selected'
Exit Edit Mode
Right click >> Set Origin >> Origin to 3D Cursor


Now, when you rotate in object mode, it should rotate and flip down.